1. Field of the Invention
The present invention relates to an image sensor which includes a plurality of sensor portions each including a photodiode, and to a production method for the image sensor.
2. Description of Related Art
Image sensors typically include a plurality of sensor portions each including a photodiode, and are capable of determining a light amount distribution with respect to the arrangement of the sensor portions. Such a photodiode is conventionally produced, for example, by implanting and diffusing a P-type impurity into a surface portion of an N-type first region defined in a semiconductor substrate to form a P-type second region. Thus, a first photodiode is provided, which includes the N-type first region and the P-type second region provided on the first region (see, for example, Japanese Unexamined Patent Publication No. 2000-312024).
Further, an N-type third region is optionally formed on the second region. In this case, a second photodiode constituted by the second region and the third region is provided in addition to the first photodiode. With this arrangement, the second region can easily be depleted by applying a reverse bias voltage to the first and second photodiodes.
When the second region is completely depleted, the photodiodes each generate maximum photovoltaic power (photoelectric current) for a predetermined light amount, and have a minimum capacitance.
The formation of the third region is achieved by diffusing an N-type impurity into a region including the second region.
In order to completely deplete the second region, the second region and the third region should each be formed as having a predetermined impurity concentration profile (as having a predetermined thickness and a predetermined impurity concentration). However, it is difficult to precisely control the impurity concentration profile by the diffusion of the impurity.